The toxic substances included in exhaust gas of an automobile using gasoline as fuel, are mainly hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), and by introducing them into a catalyst apparatus using platinum, palladium and rhodium, hydrocarbons, carbon monoxide and nitrogen oxides are oxidized or reduced to water and carbon dioxide, to carbon dioxide, and to nitrogen, respectively, and removed simultaneously. To oxidize or reduce these toxic substances efficiently, it is necessary for gasoline and air to combust completely and to be theoretical air/fuel ratio without remaining oxygen, which requires to measure always oxygen concentration in exhaust gas using an oxygen sensor or the like, so as to control fuel injection amount or the like, based on this information.
Nitrogen oxides are generated only in extremely trace amount in usual combustion, however, in a combustion chamber becoming a high temperature and high pressure state, nitrogen is easily oxidized and generation amount increases. In recent years in which compression ratio has become high in average to enhance combustion efficiency, discharged amount thereof has attained to a non-negligible level.
Among the whole discharged gas, generation amount by discharged gas from an automobile occupies 30%. Trace amount of nitrogen oxides are useful for growth of plants, however, high concentration thereof causes air pollution, photochemical smog, and acid rain, therefore, in an automobile, an engine control is performed so as to decrease compression ratio or combustion temperature to suppress generation thereof.
In addition, in a flow passage of exhaust gas of an automobile, a catalyst is arranged in response to toxic substances to be purified, so as to purify the toxic components such as nitrogen oxides in exhaust gas at one stage or in step-wise. As such a catalyst, an integral-structure-type catalyst where a catalyst composition is coated on a honeycomb-type structure has been used. The honeycomb-type structure means the one in which many fine parallel gas flow paths extend in a barrel of a structure made of a metal such as stainless steel, or a heat resistant material such as ceramic, and the catalyst composition coated on the part forming this gas flow path. Among such a honeycomb structure, the one in which both end faces of the gas flow path is open is called a flow-through-type, and the one in which either of the end faces of the gas flow path is sealed is called a wall-flow-type. In the wall-flow-type, the wall face of the gas flow path serves as a filter to filter off a particulate component such as soot from exhaust gas.
As one technology for purifying nitrogen oxides discharged from a diesel engine, there has been known denitration technology using fuel light oil as a hydrocarbon for reduction, for example, use of a catalyst in which a transition metal and a precious metal are supported on a mordenite-type zeolite carrier has been proposed (PATENT LITERATURE 1). By using this, it has become possible to efficiently reduce the nitrogen oxides under oxygen excess atmosphere in exhaust gas.
On the other hand, in purification by an automotive catalyst (three way catalyst) for a gasoline vehicle, in addition to an NO reduction reaction via a reaction under rich atmosphere represented by a steam reforming reaction or a water gas shift reaction, there is included a CO—NO reaction, which is one of important elementary reactions to reduce NO even in a lean region having relatively high oxygen concentration. The CO—NO reaction is the one to reduce NO by utilizing CO present in a large quantity in exhaust gas from an automobile, and the reaction proceeds as in the following reaction equation (1).CO+NO→CO2+N2  (1)2CO+O2→2CO2  (2)
It should be noted that in stoichiometric atmosphere, it has also been known that when temperature exceeds 600° C., the reaction (2) becomes predominant, while at 600° C. or lower, the reactions (1) and (2) proceed competitively, and selectivity thereof depends on oxygen concentration around a precious metal element (NON PATENT LITERATURE 1). However, such phenomenon has been confirmed that in the CO—NO reaction under coexistence of oxygen, because the CO—O2 reaction proceeds competitively in this way, the CO—O2 reaction little proceeds using a known catalyst.
In exhaust gas regulations of an automobile, a regulation standard is set assuming running on a practical road, and exhaust gas concentration is not the one measured under engine operating condition in a steady state. In such the regulation standard on the assumption of running on a practical road, not only high speed running with a stable combustion state but also running under condition of small accelerator opening and low engine speed in an urban area are assumed. Generally, in order to enhance catalytic activity, high temperature condition to some extend is desirable, however, because CO or HC, which is a reducing agent, reacts with oxygen selectively in a region where oxygen coexists, in particular, as for the NO reduction reaction in which oxygen is present in excess amount than stoichiometric ratio, there was sometimes the case where satisfactory purification effect of exhaust gas was not obtained.
Further, because temperature of exhaust gas exceeds 1000° C. in many cases, in particular, in a gasoline vehicle, an automotive catalyst essentially requires to have heat resistance at high temperature, in view of practical use. To solve this problem, such a catalyst for exhaust gas purification has been proposed that is provided with a precious metal particle and a substrate supporting the relevant precious metal particle, and formed with a compound between the precious metal particle and the substrate at least at a part of a contact region of the precious metal particle and the substrate (PATENT LITERATURE 2). According to PATENT LITERATURE 2, because a compound between the precious metal particle and the substrate is formed at least at a part of a contact region of the precious metal particle and the substrate, and movement of the precious metal particle is suppressed (anchor effect), sintering of the precious metal particle is suppressed and decrease in purification performance of the exhaust gas purification catalyst can be suppressed. However, although PATENT LITERATURE 2 intends to suppress sintering of the precious metal particle at a high temperature exceeding 500° C., it does not take into consideration of the above reactions (1) and (2) proceeding competitively at 500° C. or lower, about the CO—NO reaction under coexistence of oxygen, therefore purification performance of the exhaust gas purification catalyst was not sufficient.
Under such circumstances, there has been desired earnestly a denitration catalyst composition which is capable of enhancing purification ability of exhaust gas and stably purifying nitrogen oxides in exhaust gas, without increasing activated metal amount in a catalyst composition.